No debug output via TELNET. Cisco debug output via telnet/SSH.

This is the presentation of a command that it is very helpful and it should be a must know for any network administrator out there and even for some system administrators, depending how far their responsibilities go at their companies.

Do not expect for any OUTPUT of the Cisco equipment if you are connected remotely, unless you will use below command. If you are using a simple serial connection which is the first basic method of connecting to such equipment through the console port, you would get the output of the debug command by default.

You will need this command when you are trying to see some OUTPUT messages (replies) from a debug command issued on a Cisco router/switch. The use of the command is strictly needed no matter if you are connected to the device via telnet or SSH.

The command to enable the output of Cisco equipment and pass it out on your remote sessions is:

Terminal monitor

You will have to type it in enable mode prompt (privileged exec) like you can see in below steps:

R1>en

R1#terminal monitor

To disable it, you will have to type:

Terminal no monitor

For example:

R1#terminal no monitor

Now this will be very useful especially when you will be connected to a switch or a router on your network and you are trying to troubleshoot some specific error using the debug command.

Keep in mind that sometimes even if you type this command you might still don’t get any message. The reason for this might be for example, if you don’t trigger an action that would cause the equipment to detect the change.

One particular such case would be if you are trying to monitor the debug of a routing protocol using debug ip ospf for example and you don’t have ospf running or correctly configured. Another such case when you don’t get any output on your remote session monitor would be if you try to get the output for a specific process running or preparing to run on the equipment, but it wouldn’t run due to the lack of an action which the router is waiting to happen before it can act and output a confirmation or a debug message. Best way to manage to do this is to first think if you would have what to capture on your remote session as an output after you enable the terminal monitor, If you have such an action prepared or waiting to happen that can trigger any change in the behavior of the process that you are trying to capture with your debug parameter.